1. Technical Field of the Invention
The present invention relates to the production of elements comprising at least one block of dense material constituted by hard particles dispersed in a ductile binder phase, it being possible for the dense material to be enriched locally with binder phase by imbibition. The invention relates more particularly to the production of tools made of ceramic/metal composite, which is also known as cermet, and more particularly of tools for use in oil and gas and/or mine drilling.
2. Description of Related Art
Drilling tools are constituted by bits surmounted by cutters for cutting or grinding materials such as rock. The cutters, which are the active part of the tool, are in most cases made of carbide, an extremely hard but brittle material. That fragility is particularly disadvantageous when such tools are used to drill geological layers constituted by rocks of different hardness, it being possible for such heterogeneities to cause impacts which may give rise to cracks in the cutters and thus lead to wear of the bits by flaking or to breaking of the cutters.
In order to reduce the risks of premature wear or of breaking of the cutters, it is known to create bits made of cermet (ceramic metal composite), the core of which is more ductile than the outside surface (which is in direct contact with the rock). The core of the bit will thus be more resistant to impacts (zone enriched with binder phase), while maintaining a good cutting ability (zone low in binder phase, which is in contact with the rock).
In order to produce such cutters, which are known as cutters having a composition gradient or property gradient called Functionally Graded Material (FGM), it has been proposed to produce non-dense cermets having a porosity gradient and to infiltrate with a binder phase in order to improve the ductility of a zone at the core of the cermet. However, that method is ill-suited, in particular to WC-Co systems, because it leads to the partial destruction of the carbide skeleton that exists prior to the imbibition and accordingly does not allow the desired properties of the cutter to be obtained.
It has also been proposed to produce cermets having a composition gradient, with a hard outside surface and a ductile core, by the natural sintering (without the application of external pressure) in solid phase of a multi-layer element, each of the layers having a different composition. However, that method does not allow the material to be densified completely and thus must be followed by an expensive hot isostatic compaction treatment. In addition, the preparation of the cermet having a composition gradient is complex because it requires the production of a series of elementary layers which fit one into the other, each layer having a different composition. Finally, that process, which is complex and very expensive, does not allow a continuous composition gradient to be obtained. Accordingly, a cermet so obtained comprises a succession of layers having substantially different hardnesses and coefficients of expansion, leading to the risk of delamination at the interface between two adjacent layers.
In order to remedy the disadvantages of solid-phase sintering, it has been proposed to produce such materials by natural liquid-phase sintering, which allows a material having a completely dense, gradual structure to be obtained very rapidly and in a single step. However, that process has the disadvantage of weakening the composition gradient quite considerably by virtue of the migration of liquid between the layers of small thickness. Furthermore, and wholly unexpectedly, the composition gradient remains discontinuous when the dwell time in the liquid state remains below a critical time beyond which complete homogenization of the cermet is noted.
For those various reasons, the three methods which have been proposed are not suitable for the industrial manufacture of drilling tools having satisfactory use properties, both wear resistance at the surface and ductility or toughness at the core.
In addition, in order to improve the working life of cutting tools, it has been proposed to deposit hard coatings of nitride, carbonitride, oxide or boride on the surface of cermets. Such methods have been described, for example, in U.S. Pat. No. 4,548,786 or 4,610,931, the disclosures of which are hereby incorporated by reference. However, those methods have the disadvantage that they only improve the resistance of the cermet to wear by abrasion, and that improvement is achieved only over small thicknesses (several microns). Moreover, because the nature of the coating differs from that of the bit, delamination or flaking of that layer may occur following thermomechanical stress of the cutter.
It has also been proposed to improve both the wear resistance of the surface and the impact resistance of cermets of the WC-Co type by bringing a cermet that is substoichiometric in terms of carbon into contact with a carbon-rich gaseous phase (methane). Under the effect of temperature, the carbon from the gaseous phase diffuses into the substoichiometric cermet and reacts with the η phase according to the chemical reaction 2C+Co3W3C (η phase)→3WC+3Co, resulting in the release of cobalt, which migrates towards the zones that are less rich in cobalt. However, that method, which is described, for example, in U.S. Pat. No. 4,743,515 (the disclosure of which is hereby incorporated by reference), has the disadvantage that it results in a binder phase gradient that is rich in cobalt over one or two millimeters, while the core of the cermet remains fragile because it is constituted by the η phase and can easily crack during repeated impacts.
Finally, it has been proposed to produce cutting tools having specific structures, especially honeycombed structures, which have the advantage of combining good wear resistance and good toughness. Such cermets having a functional microstructure exhibit a compromise of ductile/fragile properties which is of interest but remains inadequate for the intended application. That composite material is the subject of U.S. Pat. No. 5,880,382 (the disclosure of which is hereby incorporated by reference).
There is a need in the art to remedy the foregoing disadvantages.
Imbibition is understood as being an enrichment with a liquid of a completely dense solid/liquid system in which at least a solid phase is in the form of gains able to adapt their form by absorption of liquid, thus making the system more stable energetically. The enrichment with liquid is made under the effect of the driving power resulting from the migration pressure existing in such systems.
Infiltration is an enrichment with a liquid of a non completely dense solid/liquid system under only the driving power resulting from the capillarity also named capillary pressure. An impregnation involves a third phase named non condensed phase (gaseous phase) in addition to the two condensed phases (solid/liquid).